Protein Matrix and Dielectric Effect in Cytochrome c

Abstract

The effect of the protein matrix on the standard potential of a buried redox center has been investigated by using a selection of mutants and chemical derivatives in Saccharomyces cerevisiae cytochrome c isoform 1. Assuming only local structural perturbation and no alteration of the iron-ligation chemistry, ΔEm0′ can be regarded as a measure of the difference in polypeptide solvation of the heme charge, which refiects the dielectric properties of the protein. The evaluation of an apparent dielectric constant (Uexp/Utheo) yields variable, and sometimes even negative, values if Uexp = ΔG0redox. However, some consistent result are observed if Uexp = ΔH 0redox, with a measured εΔΔ Hredox = 19 ± 6. The variability is thus attributed to an entropic factor (εΔΔSredox) that is investigated using a series of substitutions of Asn52 and/or Tyr67. In double mutants Y67F/N52I Y67F/N52V, where most of the hydrogen bond network in the heme crevice is eliminated, ΔSredox compares to the wild type. This indicates that a fully consistent hydrogen bond network has a similar polarizability as an apolar matrix. We therefore argue that the variability in net dielectric susceptibility arises from conformational polarizability, a factor that is not a function of atomic properties and coordinates and is therefore hard to predict using conventional physical relationships.